In a manufacturing method of a semiconductor device, various thermal processing apparatuses are used for subjecting an object to be processed, which is a semiconductor wafer, to an oxidation process, a diffusion process, a CVD (Chemical Vapor Deposition) process, and so on. A general thermal processing apparatus includes a thermal processing furnace that is composed of a processing vessel (reaction tube) configured to receive a semiconductor wafer and to thermally process the same, a heating resistor disposed around the processing vessel, and a heat insulation member disposed around the heating resistor. The heating resistor is arranged on an inner wall surface of the heat insulation member via support members.
In a thermal processing apparatus capable of performing a batch process, for example, there is used, as the heating resistor, a helical heating element arranged along an inner wall surface of a cylindrical heat insulation member, so that an inside of a furnace can be heated to a high temperature such as from about 800° C. to about 1000° C., for example. There is used, as the heat insulation member, a member formed by baking a heat insulation material such as ceramic fibers into a cylindrical shape, so as to assist effective heating by decreasing a heat quantity which is lost as radiant heat and conductive heat. There are used, as the support members, members made of ceramics which support the heating element at predetermined pitches such that the heating element can be thermally expanded and thermally shrunk.
In the above thermal processing furnace, in order that the heating element can be thermally expanded and thermally shrunk, the heating element, which is helically formed, is supported such that a clearance is defined between the heating element and the heat insulation member. However, by using the heating element under a high temperature, the heating element undergoes a creep strain, and slowly increases in length over time. In addition, the heating element is thermally expanded during a heating operation. Moreover, there is an apparatus that quickly cools the heating element by blowing air thereto so as to decrease a temperature thereof. Due to the repeated rise and drop in temperature, the heating element is likely to be deformed. This may generate a short-circuit between adjacent parts of the deformed heating element, which may invite disconnection.
Particularly in a vertical-type thermal processing furnace, the heating element is moved in the support members because of the repeated heat expansion and heat shrinkage caused by the rise and drop in temperature, and the heating element is moved downward little by little because of gravitation. Then, the moving amount is accumulated at a lowermost turn of the heating element. Namely, because of the accumulation of the movement of the heating element, a winding diameter of the lowermost turn is increased. When the heating element of the increased winding diameter reaches an inner surface of the heat insulation member and cannot be expanded outside any more, the heating element is then deformed in the up and down direction. Thus, there is a possibility that a short-circuit occurs between a part and another part adjacent thereto of the heating element, resulting in some disconnection by a heat of a spark generated upon the short-circuit.
In order to solve these problems, the following structure has been proposed. Namely, with a view to preventing such an accumulation to one side of the elongated heating element caused by the creep and the thermal expansion or the like, a rod-like fixing member projecting outward in a radial direction of the furnace is attached to an outside portion of the heating resistor by welding, with a fore end of the fixing member being buried to be fixed in the heat insulation member (see, JP10-233277A).
However, in the above structure in which the fixing member is joined to the outside portion of the heating resistor by welding, and the heating resistor is fixed by burying the fixing member in the heat insulation member, there is a possibility that the radial movement of the heating resistor caused by the thermal expansion and thermal shrinkage thereof is restrained. In this case, it can be considered that a stress tends to concentrate on the joined portion of the fixing member fixed on the heating resistor, which entails deterioration in durability (reduction in lifetime) of the heating resistor. Further, since the fixing member has a bar-like shape, the fixing member may easily drop out of the heat insulation member, whereby it is difficult to secure a sufficient holding force of the heating resistor, which may deteriorate the durability of the heating resistor.